The present invention relates generally to the processing of clay and, more particularly, to an improved method for concentrating a beneficiated aqueous kaolin clay slurry which includes at least partially dewatering the kaolin clay slurry via filtration through a microporous barrier, thereby eliminating clay loss commonly encountered during dewatering via conventional techniques.
Kaolin clay has many known applications in industry, including use as a filler in papermaking, a coating for paper, and a pigment in paint. However, crude kaolin clay typically contains various impurities which cause discoloration. Additionally, crude kaolin is typically too abrasive for direct use in these products. Therefore, it is necessary to beneficiate the crude kaolin clay to increase the brightness of the kaolin by removing discolorating impurities and to decrease its abrasiveness by reducing the particle size of the kaolin particles.
In general, such processes for beneficiating crude kaolin clay require that the clay be refined via wet processing as a low solids slurry. Therefore, it is necessary to add substantial amounts of water to the dry crude kaolin clay to form a clay suspension or slurry having a low solids content, generally below 50% solids by weight and typically in the range of 15% to 40% solids by weight. However, for commercial applications, the beneficiated clay slurry must have a much higher solids content. Typically beneficiated kaolin clays are shipped commercially for use in paper making, paper coating and paint making as a high solids slurry having a solids content in the range of 65% to 75% by weight. Therefore, most of the water added to the dry kaolin clay must be removed in order to concentrate the clay solids.
In conventional prior art methods for refining kaolin clay via wet-processing, the crude kaolin clay is dispersed in water, usually with the aid of a dispersing agent, to form a flowable aqueous suspension or slurry. Typically, the aqueous crude clay suspension is then subjected to a fractionation operation to remove coarse and abrasive grit. This fractionation operation is conventionally carried out by centrifugation of the dispersed aqueous clay slurry. Typically, the aqueous kaolin clay slurry is fed to the centrifuge at a solids content in the range of 45% to 50% solids. The fine particle-size fraction, generally 90% finer than 2 microns equivalent spherical diameter in particle size, is recovered as a more dilute dispersed aqueous clay slurry, typically having a solids content ranging from 30% to 40% solids by weight, while the coarser fraction is discarded.
Following fractionation, the recovered aqueous clay slurry may be passed through a magnetic collector to remove at least a portion of any iron-based impurities therefrom. Such impurities discolor the kaolin and, if not removed, reduce the brightness of the beneficiated end product. Alternatively, but usually in conjunction with and subsequent to the magnetic separation operation, the aqueous clay slurry is subjected to a bleaching step to remove insoluble iron impurities by reducing the iron therein from the insoluble ferric state to the soluble ferrous state. For such bleaching to be effective, the aqueous kaolin clay slurry must be chemically flocculated, typically by acidifying the aqueous kaolin clay slurry prior to the bleaching operation by admixing therewith an aqueous acidic solution, such as dilute sulfuric acid, in an amount sufficient to reduce the pH of the aqueous kaolin clay slurry to a level in the range of 2.5 to 3.5. Additionally, the solids content of the aqueous kaolin clay slurry is typically reduced to a level of 20% to 30% solids by weight prior to the bleaching operation. The bleaching is carried out by contacting the aqueous kaolin clay slurry with a bleaching agent. The bleached kaolin clay slurry is fully beneficiated at this point and must now be dewatered and further dried to bring the kaolin slurry to commercially acceptable levels.
To dewater a beneficiated clay slurry by conventional prior art practice, the low-solids slurry is typically first passed to a mechanical filter or a electrofilter wherein a limited portion of the water is removed from the slurry. Conventional filters customarily used to carrying out this initial dewatering step include hydrocyclones, filter presses, rotary vacuum filters, electrically augmented vacuum filters, and various electrofilters utilizing electrokinetic phenomena such as electrophoresis and electroosmosis. Such conventional dewatering equipment consumes considerable energy when utilized in commercial scale clay processing plants. Additionally, the filtrate from such conventional filters generally contains from 1% to 3% by weight of fine clay particles which should be removed from the filtrate at least for economic reasons. To recover clay fines from the filtrate, it is conventional practice to pass the filtrate to a sedimentation reservoir wherein the filtrate is held in a relatively quiescent state to permit the clay particles to settle out of the filtrate and collect as a sediment at the bottom of the sedimentation reservoir. When so processing the filtrate to remove carry over solids, various polymers are typically added to the filtrate to improve and hasten solids sedimentation. This addition of polymers to the filtrate not only increases processing costs, but also any excess polymer contained in the clarified supernant water removed from the sedimentation reservoir presents a potential pollution problem if the clarified water is transferred to an impound pond and also may prevent recycle of the clarified water for further use in the clay processing plant.
Typically, when the aqueous kaolin clay slurry is supplied to such a conventional filter, it has a low solids content ranging from 20% to 30% by weight, and the filter cake from the filter would have a solids content of about 50% to 60% by weight. Thus, the slurry would still comprise about 40% to 50% water. Further dewatering on such conventional filters is impractical due to the fine particle size of the solids in the beneficiated clay slurry. Therefore, it is necessary to resort to thermal means to further dewater the beneficiated clay slurry to a commercially acceptable solids content.
Typically, at least a portion of the partially dewatered slurry is passed through a spray dryer or other direct contact-type evaporator such as a gas-fired kiln, wherein the clay slurry is contacted with a drying medium, having a temperature of 1000.degree. F. or more, such as hot air or hot flue gas typically generated from the combustion of natural gas. It is customary to pass only a portion, typically about 30% to 50%, of the clay slurry through the spray dryer and then to remix the thoroughly dried clay slurry from the spray dryer with the remaining portion of partially dewatered slurry in a high shear mixer to produce a product clay slurry having a solids content of 65% to 75%.
A problem associated with concentrating clay slurries in spray dryers is that spray drying is a relatively inefficient process and considerable energy must be consumed in the spray drying process in order to evaporate the water in the clay slurry. In conventional prior art spray drying systems used for concentrating clay slurries, the water vapor evaporated from the clay slurry in the drying process is typically vented to the atmosphere in the exhaust gas from the dryer. Thus, the energy expended in evaporating the water vapor is wasted.
Alternatively as disclosed in commonly assigned U.S. Pat. No. 4,687,546, the partially dewatered beneficiated kaolin clay slurry from the preliminary filtering step may be further concentrated by evaporating water therefrom by passing the aqueous clay slurry through one or more non-contact evaporative heat exchangers in heat exchange relationship with a heating vapor comprising water vapor previously evaporated from the clay slurry. In this manner, an energy efficient process is provided for concentrating a beneficiated aqueous clay slurry in that use is made of the heat normally wasted when the flue gas from the spray dryer, together with the water vapor evaporated from the clay during the spray drying process, is vented to the atmosphere. Further, by using indirect heat exchange between the aqueous clay slurry and the heating vapor as a means of evaporating water vapor from the clay slurry, the formation of agglomerates typically encountered in direct contact evaporators is avoided. Additionally, as direct contact with the heating gas is avoided, no degradation in brightness is experienced.
It is an object of the present invention to provide a method for concentrating an aqueous clay slurry by forcing liquid in the slurry through a microporous barrier, such as a semi-permeable membrane, via a positive gage pressure differential without electrical assist, whereby the filtrate liquid, i.e. the permeate, removed from the aqueous clay slurry is substantially free of carry over clay solids thereby eliminating the need to further process the filtrate liquid to recover clay solids.
It is a further object of the invention, to provide a method of the foregoing character, which can effectively filter highly dilute slurries of fully dispersed fine kaolin particles, which is susceptible of operation in a continuous dynamic mode, and which can operate for relatively extended time periods without diminution of effectiveness.
A still further object of the invention is to provide a method as discussed, which can readily filter extremely fine kaolin particles, including particles having even smaller dimensions than the pore size of the microporous barrier utilized.
It is a yet further object of the present invention to provide a process for producing a kaolin clay product comprising beneficiated fine-particle size kaolin clay particles in an aqueous slurry at a commercially shippable solids level, wherein the slurry of beneficiated kaolin clay is at least in part dewatered through the use of a membrane filter, such as a polymeric ultrafiltration membrane.